1. Field of the Invention
The present invention relates to a light emitting diode (LED) driving circuit, and more specifically, to an LED driving circuit that uses a phase-controlled dimming circuit for constant current drive of an LED.
2. Description of the Related Art
Conventionally, in an illumination apparatus having an incandescent lamp as a light source for illumination, a phase-controlled dimming circuit has been widely used which uses a TRIAC (bidirectional thyrister) to perform a dimming operation. The dimming circuit includes a controller, and the controller controls a conducting angle of the TRIAC to change a duty ratio of a commercial alternating-current power supply voltage to be supplied to the dimming circuit. Accordingly, power to be supplied to the light source is changed, so that the dimming operation is performed.
Recently, also in an illumination apparatus having an LED as a light source for illumination, a dimming device including a phase-controlled dimming circuit capable of performing a dimming operation has been suggested (for example, refer to JP-A-2009-26544).
JP-A-2009-26544 describes a dimming device for an LED. The device includes a dimming circuit that controls a conducting angle of alternating current supplied from a commercial alternating-current power supply so as to phase-control an alternating current to be supplied to an LED, a rectifier circuit that rectifies a voltage waveform of an alternating current output from the dimming circuit to output a direct current voltage, a smoothing circuit that smoothes the direct current voltage output from the rectifier circuit to output the smoothed voltage to the LED, an A/D converter circuit that converts a voltage waveform of the alternating current output from the dimming circuit into a digital voltage signal, a PWM signal generation circuit that obtains a conducting angel set in the dimming circuit from the digital voltage signal and generates a PWM signal for controlling a current flowing time of the current to be supplied to the LED by first pulse width modulation (PWM) to achieve a duty cycle for dimming operation corresponding to the conducting angle, a first switching circuit that switches ON/OFF of the current to be supplied to the LED, a PWM control circuit that controls the switching operation of the first switching circuit by second PWM so as to allow constant current to flow in the LED, and a second switching circuit that causes the first switching circuit to perform an ON switching operation by the PWM control circuit during a period when a PWM waveform of the PWM signal output from the PWM signal generation circuit is high (H).
However, the dimming device that performs the LED dimming with the conventional phase-controlled dimming circuit, which is described in JP-A-2009-26544, has the following problems.
Conventionally, the dimming circuit using the TRIAC which performs the dimming operation for an incandescent lamp used for illumination in a general house has been widely spread, and the incandescent lamp tends to be replaced with an LED illumination device. Therefore, resulting from mismatch between the dimming circuit having the TRIAC and the LED illumination device, flicker or flash may occur. In other words, the dimming circuit is originally designed for the incandescent lamp as a load. Since the efficiency of the LED illumination is high, the current flowing in the TRIAC becomes smaller. Therefore, the dimming circuit may malfunction due to foreign noise or noise resulting from interference between the dimming circuits. In order to solve the problems, a method is conceivable where a bleeder current (holding current) depending on holding current characteristics of the TRIAC is caused to flow so as to keep a conducting state of the dimming circuit. However, as the bleeder current flows, power loss in a bleeder circuit of the LED driving circuit increases and the temperature thereof increases largely.